As is known in the art, RF transmitter design is centered on a design tradeoff between the linearity of the power amplifier and its efficiency. This tradeoff relates directly to the usefulness of the resulting device. High linearity results in a higher possible data rate and therefore compatibility with complex standards such as Wireless Local Area Network (WLAN) and Worldwide Interoperability for Microwave Access (WiMAX), and high efficiency allows for reduced cooling, energy usage and power supply requirements (e.g., in stationary applications), and longer use or smaller battery size (e.g., in cell phone and portable applications). The general perception that the tradeoff between linearity and efficiency is fundamental tends to produce designs that compromise between these two design goals. The resulting systems may be either linear or efficient, or are designed specifically for a single communications standard and therefore have limited flexibility of use. Meanwhile, consumer demand for both greater transmission rates and smaller devices continues to drive the need for an architecture that is capable of both linearity and efficiency.
As is also known in the art, communications standards that support high data rates such as WLAN/WiMAX employ variable-envelope modulation, and so linear amplification is required. One conventional approach is to use an inefficient but highly linear power amplifier. However, there are two main types of transmitter architectures that enable the use of more efficient but non-linear switching mode power amplifiers: (1) polar, and (2) outphasing, or linear amplification of nonlinear components (LINC).
Conventional polar architectures divide a signal to be amplified into amplitude and phase components. The phase component is used as the input to a non-linear, high-efficiency switching power amplifier, while the amplitude component drives the power supply of the power amplifier to create a varying-envelope signal. While this improves the power amplifier efficiency, it also requires the use of an efficient wide-output range, high-bandwidth power converter. Because converter efficiency degrades dramatically as bandwidth increases, it is very difficult to achieve high efficiency for high data-rate communication standards. This is exacerbated by the 5-10× bandwidth expansion that occurs during the conversion from Cartesian to polar coordinates. Thus, this conventional approach is only practical for low-bandwidth systems.
Outphasing, and specifically conventional LINC architectures, is based on the fact that an arbitrary input signal that can be divided into two constant-amplitude, phase-modulated signals that can each be non-linearly amplified and then passively recombined as a vector sum. This vector sum produces an output signal that is a linearly amplified version of the input. The LINC strategy eliminates the high-bandwidth power converter of the polar architecture, using outphasing to realize amplitude variation. However, the efficiency of the power combining is high only over a small range of output powers. To avoid signal distortion and preserve amplifier efficiency, an isolating combiner is often used. Conventional isolating combiners achieve 100% efficiency only at maximum output power. When the inputs are outphased to vary the amplitude, power is wasted as heat in the isolation resistor. The result is an overall efficiency that is inversely proportional to the peak-to-peak average power ratio (PAPR), limiting the benefits of this conventional approach in high data-rate communication standards such as WiMAX, in which the PAPR is high.
One of the major drawbacks of the LINC architecture is the power wasted in the power combiner. However, a combiner must be used to isolate outphased power amplifiers and provide a fixed impedance load to the power amplifiers in order to avoid signal distortion and preserve switching amplifier efficiency. But power is wasted as heat in the combiner resistor when the inputs are outphased to vary the amplitude. Since the power delivered to the combiner by power amplifiers is constant, the efficiency of the LINC system is directly proportional to the output power sent to a load. The time-averaged efficiency is therefore inversely proportional to the peak-to-average power ratio (PAPR). Unfortunately, high-level modulation schemes such as 64-QAM and OFDM tend to have high PAPR, leading to low average efficiency when the LINC system is used.
To alleviate the problem of wasted energy during outphasing, sometimes non-isolating combiners are used. The Chireix combiner is a prominent example which uses compensating reactive elements to enhance the power-combining efficiency. However, the Chireix combiner can only be tuned for a very small range of outphase angles. With outphase angles outside the tuned range, the load impedance presented to the power amplifiers deviates too far from the nominal value and the isolation between the power amplifier outputs becomes poor. The result is significant distortion and degraded amplification efficiency.
One proposed power recycling technique described in Zhang X., et al. “Analysis of power recycling techniques for RF and microwave outphasing power amplifiers,” IEEE Trans. Circuit Syst. II, vol. 49, no. 5, May 2002, pp. 312-320, attempts to enhance the power efficiency of the LINC architecture without giving up the simplicity of an isolating combiner. The isolation resistor is replaced with an RF-dc converter to recover the wasted power back to the power supply. While this approach has been shown to result in a significant increase in the overall efficiency, it suffers from excessive impedance variation at the isolation port and therefore incomplete isolation between power amplifiers. This can lead to excessive signal distortion and lower efficiency or even complete breakdown in the power amplifiers, particularly in those sensitive to load impedance, such as many switched-mode power amplifiers. An additional isolator can be added between the isolation port and the RF-dc converter to reduce this effect, but at the cost of added complexity and loss.